Magnetic hard disk drives, also known as Winchester drives, serve as the principal information storage component in many computer systems. A typical hard disk drive contains a stack of one or more magnetically recordable and readable disks, each made of a non-magnetic disk-shaped substrate coated with a magnetic recording medium on one or both sides. The predominant substrate of commercial drives is aluminum, but other substrates such as glass, ceramics, and glass-ceramics have also been employed.
The trend in the personal computer industry is towards more complex operating system and applications software and larger data files such as spreadsheets, databases, and graphics, resulting in a need for hard disk drives with increased storage capacity. At the same time, there is a trend towards compactness, as evidenced by the increasing popularity of notebook and lap-top computers and portable storage devices. The result is two seemingly contradictory demands on hard disk drives: increased storage capacity and smaller physical size. To exacerbate the problem, there exist de facto size and shape standards for hard disk drives, in the form of the space allocated to the hard disk drive by a computer's manufacturer: a hard disk drive manufacturer seeking to introduce a higher capacity hard disk drive as an upgrade feature for a computer has little flexibility and must design the new drive to fit into the space allocated by the manufacturer for the original-equipment drive. One way to increase storage capacity is to make the disks themselves thinner, so that a drive with unchanged exterior dimensions can contain more disks--i.e., more storage space. Since the substrate accounts for an overwhelming proportion of the thickness of a disk, this means, in turn, thinner substrates.
Ceramics and glass-ceramics have been proposed as alternatives to aluminum for making thinner substrate disks, because aluminum is comparatively not as stiff and is subject to warping. Representative disclosures include Goto et al., U.S. Pat. No. 5,567,217 (1996); Ishizaki et al., U.S. Pat. No. 5,561,089 (1996); Kawashima et al., U.S. Pat. No. 5,532,194 (1996); Nakagawa et al., U.S. Pat. No. 5,494,721 (1996); Yamakawa et al., U.S. Pat. No. 5,165,981 (1992); Kondo et al., U.S. Pat. No. 5,008,176 (1991); Alpha et al., U.S. Pat. No. 4,971,932 (1990); Yoshikatsu et al., U.S. Pat. No. 4,808,463 (1989); Wada et al., U.S. Pat. No. 4,808,455 (1989); Matsumoto, U.S. Pat. No. 4,738,885 (1988); and Wada et al., U.S. Pat. No. 4,690,846 (1996).
The compliance, or deflection per unit load, of hard disk substrates varies approximately with the cube of the thickness. This implies that, to achieve the same rigidity in a 15 mil thick disk (a proposed standard for disks in the future) as in a 25 mil disk, a 4.6-fold increase in the elastic modulus of the substrate material is required even though a thinner disk will be subject to lower inertial loads, e.g. during a shock event such as a fall. While aluminum has a modulus of about 72 GPa, current commercial glass and glass-ceramic substrates have elastic moduli of about 85 and 93 GPa, respectively. These modest increases may be insufficient to provide sufficient rigidity in 15 mil thick disks. Substantially higher elastic moduli can be achieved with ceramic materials such as alumina (380 GPa), silicon carbide (420 GPa), or mullite (220 GPa). However, these materials are more difficult to polish to the required surface smoothness. One solution is to coat a stiff substrate with a thin coating of intermediate hardness, such as glass or amorphous alumina. In this way a composite substrate with both exceptional stiffness and smooth surfaces can be achieved with machining costs similar to that of glass or glass-ceramic substrates.
During drive shock tests nickel-phosphorous (NiP) coated aluminum substrates will suffer damage from head slap at acceleration levels of less than 500 G, whereas glass-ceramic disks do not suffer damage at levels up to 1000 G. The superior performance of glass and glass-ceramic materials over aluminum is attributable to their superior hardness and higher Young's modulus. In addition, the high thermal expansion coefficient of aluminum (22.times.10.sup.-6 /.degree. C.) can result in significant warping of the substrate in drive environments where significant temperature gradients occur. Ceramic and glass-ceramic materials, typically having thermal expansion coefficients less than 10.times.10.sup.-6 /.degree. C., are more resistant to warping. This may be a key factor in maintaining data integrity at high area densities.
Ceramics however have their own limitations. Firstly, they are comparatively difficult to polish because of their grain structure, high harness, and high elastic moduli. Secondly, being insulators, it is not possible to apply an electrical bias thereto during sputtering of the tie and magnetic layers, as is done with aluminum based substrates. Thirdly, ceramics have poor static dissipation. A fourth limitation relates to the laser texturing of a landing zone on the disk, to prevent stiction of the head when it lands. Laser zone texturing of a NiP/aluminum substrate is straightforward due to its high absorbtivity and ductile nature. However, many ceramics, including currently used glasses and glass-ceramics, have a low absorbtivity for commercial infrared lasers, resulting in more difficult and less uniform texturing characteristics. In addition, laser heating of glasses will tend to locally destroy the chemically tempered zone, if it is present. These difficulties could be overcome by electrolessly coating the ceramic or glass-ceramic substrate with a thin, amorphous metallic film, preferably of NiP. However, the surface of a conventional ceramic is difficult to activate for electroless deposition. For extant glasses or ceramics an additional step of depositing a palladium activation layer prior to NiP coating is required, increasing manufacturing cost.